Investment ring heater



1940- v D. D. MlZZY ET AL 2,221,302

INVESTMENT RING HEATER Filed Feb. 13, 1940 A TTORNEY Patented Nov. 12,1940 UNITED STATES INVESTMENT RING HEATER Dan D. Mizzy and David L.Durst, Sacramento,

alif.

Application February 13, 1940, Serial No. 318,684

3 Claims.

This invention relates to a dental casting device, and especially to anelectrically operated heater designed for the reception of investmentrings and for heating the same to a predetermined temperature, thepresent application being a continuation in part of our pendingapplication entitled Inlay heater and ring, filed May 26, 1937, SerialNo. 144,878, now issued as Patent No. 2,188,915, Feb. 6, 1940.

It is common practice in making dental castings or inlays to form a waximpression or pattern conforming to the shape of the tooth cavity to befilled. After the pattern is removed from the cavity it is placed on asprue pin and then embedded in an investment material. When the materialis hardened, the wax is melted out by subjecting the investment materialto a sufficient temperature to melt and permit complete removal of thewax. The result of this operation is that a cavity corresponding to thetooth cavity is formed in the investment material, and as such is readyfor the gold or other material from which the inlay is to be cast. Thesprue hole leading to the cavity in the investment material is generallyso small that molten metal will not flow freely into it. For this reasonpressure or centrifugal force is required to force the metal into thecavity.

Various devices have been provided for this purpose, but with varyingdegrees of success, as it is usually necessary to transfer theinvestment material from a furnace or other heating device to thecasting machine before the molten metal can be poured. This transfertakes time, and very often causes a considerable temperature drop in theinvestment material, which is undesirable due to the comparatively highcoefficient of expansion or contraction of the molten metal. Plainlyspeaking, the investment material should be heated to a point of, forinstance, 125i) R, where it is expanded an amount equal to the shrinkageof the metal when it changes from a molten to a solid state. Thisobviously will not take place unless the investment material is firstheated to bring about the proper expansion and then maintained at thattemperature until the metal is actually poured.

The object of the present invention is generally to improve and simplifythe construction and operation of dental casting devices of thecharacter described; to provide a heater and investment ring which isadapted to be inserted in a centrifugal, pressure, or like castingmachine; to provide a heater and investment ring which is adapted to beelectrically heated directly in the casting machine, and maintained atthe required temperature until the very moment the molten metal is to becast; to provide a heater in which the investment ring is removable sothat a number of investment rings may be prepared and cast one afteranother in the same heater Without cooling the heater; to provide aheater and investment ring which will require a minimum of investmentmaterial and from which the investment material and the completedcasting may be quickly and readily removed; to provide an electricallyactuated heater of the character described which is brought up to apredetermined temperature in a comparatively short time, and in whichelectric current consumption is maintained at an absolute minimum; toprovide a heater which is so constructed and designed that it may bereceived and secured in practically any type of casting machine, whethercentrifugal or pressure actuated; and further, to provide anelectrically operated heater which is compact and rugged in constructionto withstand abuse of every kind.

The heater is shown by way of illustration in the accompanying drawing,in which-- Fig. l is a perspective View of the support on which theelectric resistance heating element of the heater is wound and secured;

Fig. 2 is a similar View showing the resistance heating element coveredwith insulating material, and the clamping rings and terminal posts orprongs which are connected with the opposite ends of the resistanceheating element;

Fig. 3 is a central vertical section of the heater;

Fig. 4 is a perspective view of the same;

Fig. 5 is a side elevation of the sprue former, showing the sprue pinand wax pattern mounted thereon;

Fig. 6 is a vertical section of the investment ring, showing it inposition on the sprue former, and also showing the investment materialafter it has been poured into the investment ring;

Fig. '7 is a View similar to Fig. 6, showing the sprue former and pinremoved and the investment ring ready for insertion in the heater; and

Fig. 8 is a perspective view of the investment ring.

When a dentist is working on a cavity in a tooth, and has clean d it outand prepared it for the insertion of a filling, if a gold inlay fillingis to be employed an impression is taken of the cavity with a smallpiece of wax. After the impression or pattern has been made, the patternis removed and may have the appearance of that indicated at 2 in Fig. 5.It is placed on a pin 3 which is known as a sprue pin, and this pin isin turn placed in a rubber plate known as a sprue former, which isindicated at 4. A tubular sleeve 5, known as an investment ring,terminates in a rounded head section 6, on which is formed an annularflange 6a. A thin composition of inlay investment material is pouredinto the inlay ring from the upper end, until it is full, as shown inFig. 6, and the investment-painted pattern on the sprue pin and formeris gently worked down into the investment ring and seated. Theinvestment ring and sprue former are left until the investment materialhas substantially set, and the sprue former, together with the pin, isthen removed. This leaves the wax pattern within the investmentmaterial, as shown at 9 in Fig. '7, and it leaves a passage 8 where thesprue pin has been removed.

The next operation in general practice is to place the investment in afurnace and there to heat it until the wax pattern melts and the waxflows out of the hole 8. A portion of the wax usually remains andcarbonizes on the wall of the mold, leaving a residue. The gold or othermolten metal to be employed is then poured in to fill the cavity left bythe wax pattern, and after the metal has hardened the investmentmaterial is broken out and the casting recovered and prepared for finalinsertion in the cavity of the tooth. A great deal of skill, experienceand practice is required to operate in this manner, as the casting willbe either too small or too large, depending upon the temperature towhich the investment material was heated. If it was heated to the exactor proper temperature, the casting would be of correct size, but evenso, its exterior surface has a tendency to be porous or rough, due tothe carbonized wax left within the mold cavity.

The heater forming the subject matter of the present invention isdesigned to utilize a known principle, to wit, that molten wax flowstowards a cold surface; hence if the investment ring and investmentmaterial can be gradually heated from the exterior, and not at the sprueopening end 8, the wax as it melts will flow outwardly through thecomparatively cool sprue pin opening, and will be completely dissipated,thereby preventing carbonization or the forming of carbon particles onthe inner face of the mold cavity.

The heater is best illustrated in Figs. 1 to 4 inclusive. It consists ofan inner metallic cylinder 9 about which is placed a layer IQ of anysuitable form of insulating material, such as asbestos, mica or thelike. Placed on top thereof is a thin strip of mica II, which extendsfrom end to end of the insulated cylinder 9. Perforations are formed atopposite ends of the mica strip. One end of an electric resistance wire,constructed of nichrome or the like, is passed through the upperperforations as shown at l2, and is then wound about the cylinder toform a heating coil or element l3. The lower end of the wire is passedout through the perforation M, and two free ends I2 and I4 are thusprovided. The heating element I3 is now covered with a layer of asbestosl5, or similar insulating material, and two rings l6 and H are slippedover and clamped around the insulating material. The free end of thewire I! is bent down through a notch in the insulating material andsoldered or welded to a lug l8 on one ring, and the other free end issimilarly bent over and secured to a lug IS on ring ll. Both rings arereinforced with plates such as shown at 20, and these are drilled andtapped to receive the threaded end of the pair of pins or contact prongs2| which are applied later. The rings l5 and H are made of any suitablematerial, and form conductors between the prongs 2| and the respectiveends of the heating element l3. Also, they form a sturdy support for theprongs, this being exceedingly essential as they are subject toconsiderable wrenching and bending action when an electric connector orsocket is applied, as will hereinafter be described.

' After the heating element has been prepared as shown in Fig. 2, it isagain covered with insulating material, as shown at 23, and it isinserted within an outer cylinder 24 (see Figs. 3 and 4) An upper headmember in the form of a flaring ring 25 is then introduced, and flanges26 and 21 formed thereon are welded or otherwise secured in place topermanently seal and enclose one end of the electric heating element. Aringshaped bottom plate 28 is inserted in the lower end, and flanges 29and 30 formed thereon are welded or otherwise secured to the respectivecylinders 9 and 24, thereby permanently enclosing and sealing theheating element. One side of the exterior cylinder has arectangularlyshaped opening formed therein which is bridged by a clampgenerally indicated at 3|. A rectangular block of insulating material isslipped between the clamp and the cylinder, and its face when insertedis exposed by forming a rectangularly-shaped opening in the clamp. Theinsulating block has two holes formed therein which register with thethreaded openings formed in the reinforcing plates 20 on the rings l6and H. The prongs or pins 2| are inserted through the openings andscrewed into the tapped reinforcing plates, and the heater is thuscompleted. The heater when completed is exceedingly compact and ruggedin construction, its actual size being surprisingly small, for instanceit is only 1% inches in diameter and has an overall length of 1 inches.This shape, size and length makes it suitable for insertion inpractically any type of casting machine, whether centrifugally orpressure actuated.

The heater is in actuality a small furnace, and does away with thelarger furnaces in general use at present. The current consumption is sosmall as to be almost negligible, and at that it assumes the requiredtemperature, which in actual practice is about 1250 F., in fifteenminutes or less. The heater has the advantage that it may be placeddirectly in the casting machine with the investment ring mountedtherein. Current is applied to the heater by merely placing a socket orconnector, such as shown in dotted lines in Fig. 4, over the prongs. Asthe heater heats up, the heat is applied to the exterior of theinvestment ring and around the cylindrical portion 5 thereof. Theinvestment material is thus heated from the exterior inwardly andcomparatively little if any heat is applied at the ends; as the naturaltendency of melting wax is to flow toward a cold surface, it will flowfreely out of the sprue opening 8 as. the temperature increases, andwill be completely dissipated before carbonization can take place.

The number of turns and the size of wire used in the heating element I3is such that a temperature of 1250 F. will be reached quickly, and theinvestment material will be maintained at that temperature, conductionand convection losses being such that that temperature will not beexceeded. Thus when the molten gold is ready to be poured or cast, it isonly necessary to pull or remove the socket and to start the castingmachine in operation. The moment the gold has been poured, whethercentrifugally or otherwise,

it hardens almost immediately. The heater and investment ring may thusbe removed, and a second investment ring inserted, this beingaccomplished without the cooling of the heater. The cast investment ringmay be plunged into cool water, and thus the investment material may beimmediately broken out and the casting recovered. When it is recoveredit will be of exact size, as the investment material has been heated tothe exact temperature required; and the casting will be smooth infinish, as no carbon particles were formed during the heating of theinvestment material.

The several parts forming the heater are made of metal, and these arewelded with relation to each other, thus making a strong, rigid housingor body, and as the prongs or terminal ends of the heating element aresupported at their inner ends by the threaded connection formed betweenthe rings l 6 and I1, and are supported also by the insulating blockinserted within the clamp 3|, they are so rigidly supported that anywrenching or bending to which they may be subjected when applying orremoving the socket member will neither tend to bend or break theelectric connections. In plain words, the heater may be subjected tounusual abuse without detrimental effect.

While certain features of our invention have been more or lessspecifically described for purposes of illustration, we wish itunderstood that changes may be resorted to within the scope of theappended claims and that the materials and finish may be such as theexperience of the manufacturer may dictate and other conditions demand.

Having thus described our invention, what we claim and desire to secureby Letters Patent is:

1. An electric heater of the character described comprising an innercylindrical tube, a heating element in the form of a helical wire coilsup ported on the tube and insulated therefrom by insulating materialexterior of the coil, a pair of clamping and current-conducting ringssurrounding and securing the last named insulating material, aconnection between each end of the wire coil and the respective clampingrings, an exterior cylindrical tube, a head member at each end securedto the tubes and closing the space between the tubes, and a pair ofspaced contact pins extending through the exterior tube and insulatedtherefrom, said pins being secured at their inner ends to the clampingand conducting rings to supply electric current to the helical heatingcoil.

2. An electric heater of the character described comprising an innercylindrical tube, a heating element in the form or" a helical wire coilsupported on the tube and insulated therefrom by insulating materialexterior of the coil, a pair of clamping and current-conducting ringssurrounding securing the last named insulating material, a connectionbetween each end of the wire coil and the respective clamping rings, anexterior cylindrical tube, a head member at each end secured to thetubes and closing the space between the tubes, a pair of spaced contactpins extending through the exterior tube and insulated therefrom, saidpins being secured at their inner ends to the clamping and conductingrings to supply electric current to the helical heating coil, a clampsecured on the exterior face of the exterior tube, a block of insulatingmaterial secured thereby, said block having a pair of holes formedtherein through which the contact pins extend, said insulating blockforming a guide and support for the contact pins to increase theirrigidity and to prevent bending or displacement of the same.

3. An electric heater of the character described comprising a pair ofinner and outer concentrically positioned metal tubes having an annularspace formed between them, a ring-shaped head secured to the respectiveends of the tubes and forming a closure for each end of the annularspace, an electric resistance wire in the form of a helical coildisposed in the annular space, a pair of combination clamping andconductor rings surrounding the helical coil, said coil and rings beingelectrically insulated from each other and the tubes, an electricconnection between each end of the helical coil and the conductor rings,a pair of contact pins secured at their innor ends one to each conductorring, the outer ends of said pins projecting through the outer tube, anda block of hard material secured on the exterior tube and havingopenings formed therein through which the contact pins extend, saidblock insulating the pins from the exterior tube, and forming a supportfor the outer ends of the pins to prevent displacement or bending of thesame.

DAN D. MIZZY. DAVID L. DURST.

